Recent technical advances have remarkably reduced the size and weight of communication devices, such as a mobile phone. Examples of a filter for use in such a communication device include elastic wave devices, such as a surface acoustic wave device and an elastic boundary wave device that can be downsized. In mobile communication systems, the number of simultaneous transmission and reception systems, such as code division multiple access (CDMA), is rapidly increasing, which rapidly increases the demand for duplexers. Further, in recent years, a large number of duplexers whose reception ends perform balanced operation have been used.
Under these circumstances, a multimode elastic wave device that has an unbalanced-balanced conversion function is used as the filter on the reception side of a duplexer. Further, a change in the mobile communication system makes requirement specifications for duplexers stricter. This creates a need for a multimode elastic wave device that has a broader passband with a steeper attenuation characteristic closer to a square shape than conventional ones.
As conventional arts of the present invention, techniques disclosed in Patent Literature 1 through Patent Literature 3, for example, are known. Patent Literature 1 discloses a technique for changing the periods of the reflector electrodes, according to patterns, in a multimode elastic wave device that includes three interdigital transducer (IDT) electrodes. This technique aims to suppress spuriousness and provide high steepness at the same time.
Patent Literature 2 discloses a technique for using a plurality of reflector groups having different periods as reflectors in a multimode elastic wave device that includes three IDT electrodes. This technique aims to suppress spuriousness in proximity to the passband and provide an excellent attenuation characteristic.
Patent Literature 3 discloses a technique for devising narrow-pitch parts in the boundaries between IDT electrodes in a multimode elastic wave device that includes five IDT electrodes. This technique aims to enhance steepness in proximity to the passband. Suppose that the narrow-pitch parts in the boundaries between the IDT electrodes are a first narrow-pitch part through a fourth narrow-pitch part in order. The electrode-finger pitch in the first narrow-pitch part is smaller than the electrode-finger pitch in the second narrow-pitch part, and the electrode-finger pitch in the fourth narrow-pitch part is smaller than the electrode-finger pitch in the third narrow-pitch part. This configuration controls the displacement distribution in which elastic waves are exited.